1. Field
This application relates to solid-state laser, specifically high-power diode end-pumped solid-state laser and its UV and deep-UV harmonic generations.
2. Background
Diode-pumped solid-state laser has become more and more attractive due to its compactness, high-efficiency, and long lifetime (also see U.S. Pat. Nos. 4,942,586 4,656,635). End pumping has received even more attention recently due to several additional advantages (also see U.S. Pat. No. 4,794,615): (1) Easy to do the mode matching between the pump and the laser beam, resulting in good laser beam quality; (2) High optical efficiency. An end-pumped Nd-doped (such as Nd:YAG, NdWO, or NdYLF) laser can reach an optical efficiency of 20% to 60%; (3) Compact and easy to maintain.
High-power solid-state UV and deep-UV laser is greatly needed in both academic researches and industrial applications, such as particle ionization experiment, filamentation experiment, fuel spray, material processing, lithography, photo-refractive medical surgeries, and so on. However, the power level of current commercial diode-pumped solid-state UV laser is still very limited (For example, see Coherent brochure, the highest 266 nm laser can only output 3 W).
High-power diode end-pumped solid-state UV laser can be achieved only when it is optimized in every aspect. This invention comprises the following four aspects:
(1) High-power fiber-coupled laser diode pumps. High-power fiber-coupled laser diodes (see Coherent Brochure, FAP800 laser diode, 35 Watts or higher) are available only recently. In this invention, high-power fiber-coupled laser diodes are used as the end pump. In addition, in this invention, a high-power pump scheme is also provided which combines multiple low and medium power laser diodes using a fiber combiner.
(2) Treatment of thermal transfer of the laser crystal. In a high-power diode end-pumped solid-state laser, only 20% to 60% of the pump power is converted into laser power. A large portion of the rest pump power is converted into heat and high temperature is resulted in the laser crystal, which could lead to thermal cracking of the laser crystals if the heat is not properly treated. A laser crystal mount with excellent heat transfer and circulating cooling mechanism is extremely important. In this invention, a laser crystal mount is designed, which provides the excellent thermal contact by mounting the crystal from diagonal direction in addition to the use of high thermal conductivity material and water circulating cooling.
(3). Designing of the fundamental laser cavity. The designing of high-power solid-state laser cavity is dramatically different from that of the low or medium power solid-state laser cavity. Due to the high-power pumping and mode matching requirement, the beam size at where the laser crystal is located has to be large (usually about 0.8 mm or larger in diameter). On the other hand, high-efficiency intracavity harmonic generation requires a small laser beam size at where the nonlinear harmonic crystals are located. Therefore, the fundamental cavity designing will not be satisfied by just using a simple cavity commonly used in the low or medium-power diode end-pumped solid-state laser, in which the beam size is more or less uniform through the entire fundamental laser cavity. Furthermore, due to the high-power pump, the laser crystal will act like a lens, which is the so-call thermal lens, or thermal lensing effect (see also U.S. Pat. No. 5,410,559). In a high-power diode end-pumped solid-state laser, such effect can be very strong and the thermal lens can be very short. In this invention the fundamental laser cavity is designed in such a way that the fundamental laser beam size is nonuniform through the laser cavity: the beam size is big at where the laser crystal is located and is small at where the nonlinear harmonic crystals are located, and the following criteria are satisfied at the same time: (1) a large laser beam size in the laser crystal allows a large pump beam size which prevents the thermal cracking and weakens the thermal lens effect at the same time; (2) a small beam size in the harmonic crystals enables a high fundamental power intensity and thus the high-efficiency and high-power harmonic generations. In addition, in this invention the strong thermal lens is carefully compensated by using the predetermined nearby curved pumping mirrors.
(4) Intracavity and extracavity harmonic generations. High intensity is required for efficient harmonic generations. Intracavity is a major way to do so, which had been discussed about 40 years ago (see paper: IEEE J. Quan. Electr. QE-6, 215-223, 1970). However, currently the power level of UV and deep-UV harmonic generations is limited by the following difficulties: (a) high fundamental power is not available, (b) most of the commonly used harmonic crystals have absorption at the UV and deep-UV wavelengths, which leads to low harmonic generation efficiency and possible damage of the harmonic crystals, (c) it is difficult to optimize the fundamental cavity and the harmonic generation cavity at the same time (d) most of the harmonic crystals are hydroscopic and the protection coating at the UV or deep-UV wavelength is easy to be damaged. This invention focuses on solving all these problems, resulting in a high-power diode end-pumped solid-state laser and its UV and deep-UV harmonic generations.